The present invention relates to a mass spectrometer, and more particularly, to a mass spectrometer with an ionization device utilizing an ionization method which uses thermoelectrons, such as electron ionization (EI) or chemical ionization (CI).
In the mass spectrometer, there is used an ionization device using various kinds of ionization methods, such as electron ionization (EI) and chemical ionization (CI). For example, in the EI, a heating current is supplied to a filament to emit thermoelectrons from the filament, and at the same time, an adequate potential difference is produced between the filament and a trap electrode (also called as an electron collector or a target) to provide a kinetic energy to the thermoelectrons. Accordingly, the thermoelectrons emitted from the filament fly toward the trap electrode, and when the thermoelectrons contact sample molecules in an ionization chamber located on the way to the trap electrode, electrons are discharged or beaten out from the sample molecule, so that the sample molecules become positive ions. Normally, the number of the electrons captured at the trap electrode depends on the number of the electrons emitted from the filament. Thus, in order to provide a predetermined value to a trap current which flows owing to the thermoelectrons reaching the trap electrode, the heating current flowing through the filament is subjected to a feedback control, so that an amount of the thermoelectrons generated in the filament becomes approximately constant, resulting in achieving the stable ionization.
In the ionization which utilizes the flow of thermoelectrons, in case the filament and the trap electrode are not adequately attached to predetermined positions with respect to the ionization chamber for operating the ionization, even if a maximum heating current is supplied to the filament, the trap current does not reach the desirable value, so that the control of the heating current as described above can not be made. Therefore, in the conventional mass spectrometer, a value of the trap current is monitored, and in case the trap current does not reach the predetermined value in the condition that the filament is lighted up to operate the feedback control of the heating current, it is determined to be an abnormal condition, s that, for example, an error is displayed.
As the causes of the aforementioned abnormality of the control of the heating current, there can be considered various causes, such as a displacement or shift of the attachment position of the filament or the trap electrode, a contact between the filament or the trap electrode and the ionization chamber, or a burnout or cutout of the filament. However, n the aforementioned conventional mass spectrometer, it can be only detected that the trap current is not normal, and the cause of abnormality can not be specified. Thus, in order to check the kind of abnormality, there is required an operation of returning an inside of a vacuum chamber of the mass spectrometer, which is kept in a vacuum condition, to an atmospheric pressure, and checking the inside thereof by a visual observation. Therefore, the operation for checking the cause of the abnormality and readjustments operation are very cumbersome.
Also, in the aforementioned structure, if the heating current does not flow through the filament, it is impossible to check whether it is abnormal or not. However, in the abnormal condition, if it is tried to supply the heating current to the filament to operate the feedback control, an excessive current flows through the filament or a part of the circuit, possibly resulting in a further breakdown.
The present invention has been made to solve the aforementioned problems, and an object of the invention is to provide a mass spectrometer having an ionization device, which can diagnose a part of abnormalities of a filament or a trap electrode before an electric current is supplied to the filament, and can perform a further detailed diagnoses of an abnormal portion in the condition that the electric current flows through the filament.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To achieve the above object, the present invention provides a mass spectrometer provided with an ionization device. The ionization device comprises: an ionization chamber disposed in a vacuum chamber; a filament for emitting thermoelectrons for ionizing gas molecules in the ionization chamber; a trap electrode, which accelerates the thermoelectrons by a potential difference between a potential of the filament and that of the trap electrode and captures the thermoelectrons passing through the ionization chamber; first current measuring means for measuring a trap current caused to flow by the thermoelectrons reaching the trap electrode; second current measuring means for measuring a total current as an entire current flowing by the thermoelectrons emitted by the filament; current control means for controlling a heating current flowing through the filament so that either the trap current or the total current has a predetermined value; first abnormality diagnosis means, which allows first and second current measuring means to measure respective current values in a condition that the filament is not energized, and which determines an abnormality based on the measured current values; and second abnormality diagnosis means, which allows the first and second current measuring means to measure respective current values in a condition that a predetermined control is carried out by the current control means, and which determines an abnormality based on the measured current values.
In the ionization device of the mass spectrometer according to the invention, the total current at least includes, in addition to the trap current, a current flowing through the ionization chamber by the contact of the thermoelectrons emitted from the filament with the ionization chamber, instead of the trap electrode. Accordingly, for example, the second cur-rent measuring means measures a current flowing through a bias voltage source which biases the filament to a negative potential with respect to the trap electrode and the ionization chamber, so that the total current can be obtained.
The first abnormality diagnosis means measures the trap current and the total current in the condition that the filament is not energized. In this case, since the thermoelectrons are not generated in the filament, both the trap current and the total current are supposed to be zero. However, since the predetermined potentials different from a potential (normally, a ground potential) of the ionization chamber itself are applied to the filament and the trap electrode, if the filament or the trap electrode contacts the ionization chamber, the trap current or the total current flows. Thus, in case the measured trap current and the total current are not zero, the first abnormality diagnosis means determines that there is an abnormality due to the contact between the trap electrode or the filament and the ionization chamber.
The second abnormality diagnosis means measures the trap current and the total current in the condition that the filament is energized and the heating current is controlled by the current control means. Although there is an upper limit to a heating current which can be supplied to the filament, in the normal condition, the current control means can stabilize the trap current at the substantially predetermined current in the condition that the heating current having the value smaller than the upper limit value is supplied. However, if the attachment position of the filament or the trap electrode is shifted or displaced largely, a proportion that the thermoelectrons emitted from the filament reach the trap electrode is extremely reduced (in other words, many of the thermoelectrons reach the ionization chamber), so that the trap current does not increase even if the heating current is increased. Therefore, even if the heating current of the upper limit value flows through the filament, the trap current does not reach the predetermined value. Thus, in case the measured trap current is smaller than, for example, the predetermined value, the second abnormality diagnosis means determines that there is an abnormality due to the shift of the attachment position of the trap electrode or the filament. Also, in case the filament is burned out or cut out, the heating current does not flow, so that the thermoelectrons are not generated, resulting in that both the trap current and the total current become zero. Thus, in case the trap current and the total current are zero, the second abnormality diagnosis means determines that the filament is burned out or cut out.
If there is a structure such that an existence of abnormality and abnormal portion are warned to a user by a display or sound based on the diagnosis results of the first and second abnormality diagnosis means, the user can recognize the abnormal portion and promptly take the necessary procedure. Also, if there is a structure such that the abnormality diagnosis is carried out by the second abnormality diagnosis means only in case the first abnormality diagnosis means determines that the condition is normal, in case the filament or the trap electrode contacts the ionization chamber, the filament can be prevented from being energized.
According to the mass spectrometer o the invention, the abnormal portion of the ionization device and the cause thereof can be detected in detail, and the results thereof can be informed to the user. Thus, the user can promptly inspect the abnormal portion and an appropriate operation, such as readjustment or replacement, can be started in accordance with necessity. Since these operations can be carried out promptly, an analysis operation becomes efficient. Also, according to the mass spectrometer of the invention, before the filament is lighted up, the abnormality due to the contact between the filament or the trap electrode and the ionization chamber can be detected and informed to the user. In this abnormal condition, if the filament is lighted up, the excessive current might flow through the circuit. However, in the present invention, this abnormality can be found without lighting up the filament, so that the undesirable damage to the circuit due to the excessive current can be avoided.